Method and electrolytes for anodizing titanium and its alloys

ABSTRACT

Titanium and its alloys are anodized by making the metal the anode in an electrolytic system which utilizes carbon as the cathode and an electrolyte consisting of water, both a silicate and a perborate of an alkali metal, the hydroxide of an alkali metal, and the alkali metal salt of an acid selected from the group consisting of ethylenediamine tetraacetic acid, diethylenetriamine pentaacetic acid, and mixtures thereof. Direct current applied to the system may be varied between 10 and 50 amperes per square foot of anode surface, with EMF pressures not exceeding a rate of 8 volts per minute until the current threshold is obtained.

United States Patent Kendall [451 May 16, 1972" Appl. No.: 838,342

US. Cl ..204/56, 106/1 Int. Cl. ..C23b 11/02 Field of Search ..204/56, 58, 38 A, 35 N;

References Cited UNITED STATES PATENTS 2/1971 Vierow ..204/56 3/1970 Mizushima ..204/56 McGraw et a1. ..204/56 De Boer et a1 ..317/230 [57] ABSTRACT Titanium and its alloys are anodized by making the metal the anode in an electrolytic system which utilizes carbon as the cathode and an electrolyte consisting of water, both a silicate and a perborate of an alkali metal, the hydroxide of an alkali metal, and the alkali metal salt of an acid selected from the group consisting of ethylenediamine tetraacetic acid, diethylenetriamine pentaacetic acid, and mixtures thereof. Direct current applied to the system may be varied between 10 and 50 amperes per square foot of anode surface, with EMF pressures not exceeding a rate of 8 volts per minute until the current threshold is obtained.

7 Claims, N0 Drawings METHOD AND ELECTROLYTES FOR ANODIZING TITANIUM AND ITS ALLOYS This invention relates to the anodizing of titanium and its alloys and more particularly to a method and to electrolytes which can effectively be used to place an electrically insulative film on such metal.

If titanium or one of its alloys is placed in direct contact with metals such as aluminum or magnesium, the structural integrity of the latter will be degraded by the voltaic action produced by the dissimilar metals. Hence it is necessary to place some electrically nonconductive material between the titanium and another metal that is joined thereto. Although paint or coatings of various plastics can be employed for this purpose, such materials add considerable weight to a structure, are expensive to apply, and are not entirely reliable. Thus there are many applications for the invention disclosed herein, which enables a lightweight and dielectric surface film to be economically placed on parts formed of titanium and its alloys.

Briefly stated, the present invention forms a strongly adhered anodic film on a titanium workpiece by making the latter the anode in an electrolytic system wherein the cathode is formed of carbon and the electrolyte is an aqueous solution containing a silicate and a perborate of an alkali metal, the hydroxide of an alkali metal, and the alkali metal salt of ethylenediamine tetraacetic acid (EDTA), or the alkali metal salt of diethylenetriamine pentaacetic acid (DTPA), or mixtures of the latter two compounds. Contaminants on the surface of the workpiece are first removed by means of the cleaner disclosed in U.S. Pat. No. 3,379,645, and then surface oxides are removed by means of the electrolytic descaling process disclosed in U.S. Pat. application Ser. No. 600,362, now U.S. Pat. No. 3,468,774, filed by Earl W. Kendall on Dec. 9, I966 and assigned to Rohr Corporation, Chula Vista, California, the assignee of the present application.

After the workpiece is removed from the bath used in the descaling process and water rinsed, it is immediately placed in the aforedescribed electrolyte containing alkali metal compounds, and a source of direct current is connected to the workpiece and the carbon cathode. A direct currentis applied to the system in which current densities of between and 50 amperes per square foot may be employed. EMF pressures are not to exceed a rate of 8 volts per minute until the threshold current density is obtained. At this point in time the system is permitted to operate for from 10 to 60 minutes with no electrical adjustments required to compensate for increased anode surface electrical resistance. The foregoing process forms an anodic film on the workpiece which is virtually electrically nonconductive and which thus precludes voltaic action between the workpiece and a dissimilar metal joined to it in a structure such as an airframe assembly. Furthermore, the film has a porous structure which makes it an excellent paint base or matrix for solid film lubricants of the nature of molybdenum disulfide.

One of the important factors in anodizing titanium and its alloys in accordance with this invention is the pH of the electrolyte, which must be at least 10.5 and preferably should be in the range of l 1.0 to 11.5 in order to prevent the conversion of the alkali metal silicate of the electrolyte to insoluble silicic acid. The necessary high pH is provided by the inclusion in the electrolyte of either one or both of the aforementioned alkali metal salts of ethylenediamine tetraacetic acid and diethylenetriamine pentaacetic acid, and an alkali-metal hydroxide. The EDTA or DTPA component of the electrolyte also serves as a sequestering, or chelating, agent which combines with stray metallic ions in the electrolyte and thus prevents the same from interfering with the anodizing process. Such stray metallic ions may include titanium or alloying metals of the workpiece which is anodized, or impurities contained in the components of the electrolyte. The electrolyte continuously absorbs carbon dioxide from the atmosphere, and since this gas lowers the pH of the solution it is necessary to add additional amounts of the alkali-metal hydroxide in order to maintain the proper pH level. Oxygen for the anodizing process is furnished by the alkali metal silicate and the alkali metal perborate contained in the electrolyte. Because of cost considerations, sodium meta silicate and sodium perborate are preferred for these compounds, but any of the soluble alkali metal silicates and perborates can be employed, such as those of potassium. For the same reason, the sodium salt of either ethylenediamine tetraacetic acid or diethylenetriamine pentaacetic acid is preferred as the compound which provides proper pH level for the electrolyte and which serves as the sequestering agent therein, although the same substitution of lithium or potassium may be made for this component of the electrolyte. The anodizing process of the invention can be performed with the electrolyte maintained at normal ambient temperatures within the range of about 65 to F. Higher temperatures permit expulsion of some of the perborate from solution and hence should not be used.

The manner in which voltage is applied to the workpiece as the anode and the carbon electrode as cathode has been found to be an important factor in anodizing titanium or its alloys in accordance with this invention. When voltage is applied at too high a rate, the thin film which is deposited on the workpiece at a lower voltage level appears to be wiped away. However,

. when voltage is applied as stated hereinbefore at a rate not to exceed 8 volts per minute until the maximum predetennined current density is obtained and the system permitted to operate for from 10 minutes to 60 minutes with no electrical adjustments to compensate for anode surface resistance, the disclosed electrolytic system produces an anodic film which is uniformly deposited on a workpiece and which possesses the physical characteristics which have been previously mentioned herein. Furthermore, since the use of a metallic electrode would introduce harmful metal ions into the electrolyte, the cathode must be formed of dense carbon and, to bring about deposition of the anodic film on the titanium workpiece, the ratio of its area to that of the workpiece should be approximately 2 to l.

The electrolytic process of this invention can be properly performed only if the surface of the workpiece to be anodized is completely clean and the oxides which are normally present on titanium and its alloys are removed. Preferably a workpiece is prepared for anodizing by first cleaning it by means of the method and film removing composition disclosed in the aforementioned U.S. Pat. No. 3,379,645. In accordance with the procedure set forth in this patent, the dry workpiece is immersed for a period of from 5 to 30 minutes in a bath consisting of from 15 to 25 parts by weight of chromic acid and from to 225 parts by weight of sulfuric acid. The workpiece is rinsed with water after it is removed from the cleaning bath and oxides are then removed from its surface by means of the descaling process disclosed in the afore-mentioned U.S. Pat. application Ser. No. 600,362. The process involves making the workpiece the anode in an electrolytic system wherein the electrolytic consists of acetic, sulfuric and hydrofluoric acids, an amide-acetylenic alcohol mixture as an inhibitor, and a wetting agent. Since titanium and its alloys are rapidly oxidized when exposed to air, the workpiece is rinsed with water immediately after it is removed from the descaling electrolyte and is then transferred to the electrolyte of this invention while still wet.

It has been found that the anodizing of titanium or its alloys in accordance with this invention can best be accomplished by use of electrolytes included in the following weight percent ranges of components:

A typical illustration of the results obtained by the above described anodizing method and electrolytes is shown by the following test data. A surface of Titanium 6A1 4V was prepared by the use of an electrolyte containing sodium meta silicate, sodium perborate, sodium hydroxide, the tetra sodium salt of ethylenediamine tetraacetic acid, and water, the foregoing components being present in the electrolyte in the ranges of weight percentages which are given in the above table. By following the aforestated electrolytic steps, the surface of the titanium workpiece was anodized. A coating of paint was then applied to the surface of the test specimen and baked. An area of the specimen was lambda-scored and the specimen was placed in a humidity chamber at 100 F. and 100 percent relative humidity. After 24 hours the specimen was removed, dried, and tape tested for adhesion of the paint coating. As no trace of paint was removed from the specimen, it was shown that the adhesion of paint to the anodized surface was excellent.

Adhesives applied to metal surfaces anodized by this process produced acceptable bond strengths as determined by shear tensile tests, and retention of solid film lubricants within the porous structure of the film produced by this process is excellent. Furthermore, anodized surfaces of titanium produced by this process develop resistances in the order of 400 to 1,000 megohms.

The embodiments disclosed herein therefore are to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

What is claimed as new and useful and desired to be secured by US. Letters Patent is:

1. An anodizing electrolyte consisting of:

about 1.0 to 5.0 weight percent of an alkali metal silicate;

about 1.0 to 3.75 weight percent of an alkali metal perborate;

about 1.0 to 5.0 weight percent of an alkali-metal hydroxide;

about 1.0 to 5.0 weight percent of an alkali metal salt of an acid selected from the group consisting of ethylenediamine tetraacetic acid, diethylenetria'mine pentaacetic acid, and mixtures of said acids; and

about 8 l .25 to 96 weight percent of water.

2. The electrolyte defined in claim 1 wherein said silicate is sodium meta silicate, said perborate is sodium perborate, and said salt is the tetra sodium salt of ethylenediamine tetraacetic acid.

3. A method of anodizing titanium and its alloys which comprises making the metal the anode in an electrolytic system wherein carbon is the cathode and the electrolyte has the following composition:

about l.0 to 5 .0 weight percent of an alkali metal silicate;

about 1.0 to 3.75 weight percent of an alkali metal perborate;

about 1.0 to 5.0 weight percent of an alkali-metal hydroxide;

about 1.0 to 5.0 weight percent of an alkali metal salt of an acid selected from the group consisting of ethylenediamine tetraacetic acid, diethylenetriamine pentaacetic acid, and mixtures thereof; and

about 81.25 to 96 weight percent of water.

4. The method defined in claim 3 wherein said silicate is sodium meta silicate, said perborate is sodium perborate, said hydroxide is sodium hydroxide, and said salt is the tetra sodium salt of ethylenediamine tetraacetic acid.

5. The method defined in claim 3 wherein the EMF applied to said electrolytic system is not to exceed 8 volts per minute until a predetermined current of between 10 and 50 amperes per square foot of anode surface is obtained, at which point in time said predetermined current is permitted to flow for from ID to 60 minutes.

6. The method defined in claim 3 wherein the ratio of the maintained at a temperature of between 65 to 100 F. 

2. The electrolyte defined in claim 1 wherein said silicate is sodium meta silicate, said perborate is sodium perborate, and said salt is the tetra sodium salt of ethylenediamine tetraacetic acid.
 3. A method of anodizing titanium and its alloys which comprises making the metal the anode in an electrolytic system wherein carbon is the cathode and the electrolyte has the following composition: about 1.0 to 5.0 weight percent of an alkali metal silicate; about 1.0 to 3.75 weight percent of an alkali metal perborate; about 1.0 to 5.0 weight percent of an alkali-metal hydroxide; about 1.0 to 5.0 weight percent of an alkali metal salt of an acid selected from the group consisting of ethylenediamine tetraacetic acid, diethylenetriamine pentaacetic acid, and mixtures thereof; and about 81.25 to 96 weight percent of water.
 4. The method defined in claim 3 wherein said silicate is sodium meta silicate, said perborate is sodium perborate, said hydroxide is sodium hydroxide, and said salt is the tetra sodium salt of ethylenediamine tetraacetic acid.
 5. The method defined in claim 3 wherein the EMF applied to said electrolytic system is not to exceed 8 volts per minute until a predetermined current of between 10 and 50 amperes per square foot of anode surface is obtained, at which point in time said predetermined current is permitted to flow for from 10 to 60 minutes.
 6. The method defined in claim 3 wherein the ratio of the area of said cathode to said anode is about 2 to
 1. 7. The method defined in claim 3 wherein said electrolyte is maintained at a temperature of between 65* to 100* F. 